1. Field of the Invention
The present invention generally relates to magnetoresistive elements for reproducing magnetic information recorded on magnetic recording media with high precision, and, more particularly, to a magnetoresistive element having a CPP (Current Perpendicular to Plane) structure that gives sense current in the thickness direction of the magnetoresistive element.
2. Description of the Related Art
The CPP-type magnetoresistive element having the sense current flowing in the film thickness direction characteristically increases the element output as the size of the element decreases. Such a CPP-type magnetoresistive element is expected to serve as a highly sensitive reproduction element for magnetic recording elements that have dramatically higher densities in recent years.
In a magnetoresistive element using a spin valve film or a tunnel junction film, the magnetizing direction of the free layer changes with a signal magnetic field transmitted from a magnetic recording medium. As the magnetizing direction of the free layer changes, the relative angle between the magnetizing direction of the free layer and the fixed magnetizing direction of the pinned layer also changes. The magnetoresistive element detects the relative angle as a change in magnetoresistive element.
In the CPP-type magnetoresistive element, the sense current is given in the film thickness direction by terminal electrodes arranged in contact with the upper and lower surface of the magnetoresistive effect film. A change of magnetoresistance is then detected so as to reproduce (or read) precisely a signal magnetic field transmitted from a magnetic recording medium. In the CPP-type magnetoresistive element, the smaller the area of the element in the direction perpendicular to the film thickness direction in which the sense current flows, the greater the change in resistance. In other words, the smaller the sense current flowing area (the section area), the greater the change in resistance. As the change in resistance becomes greater, the output of the element increases.
In a dry etching method using a conventional photolithography technique, however, one side of the above section area can be reduced to 100 nm at the smallest.
To break this limit on the minuteness, a magnetoresistive element in which a mixed layer made up of a metal and an insulating material covers the outside of the magnetoresistive effect film has been suggested. This mixture layer focuses the sense current to the metallic portion of the mixed layer. In this magnetoresistive element, the sense current path becomes smaller at the metal portion, and, therefore, the sense current path in the magnetoresistive effect film is made smaller than the section area of the physical element, so as to increase the output.
In the above structure, the sense current path running in the magnetoresistive effect film is uniformly reduced to increase the output of the element, but the resistance change ratio (MR ratio) of the element cannot be increased sufficiently. In other words, since the element resistance is also increased in this structure, the sense current value is restricted due to the heat generation from the element, and a further increase in output cannot be expected.
The inventors of the present invention have studied a technique for reducing the sense current flowing in the magnetoresistive effect film. To reduce the sense current, an oxide layer is inserted as a current path control layer in a part that contributes to a resistance change in a CPP-type magnetoresistive effect film, so that the sense current path can be reduced in size. By this method, the MR ratio can be increased with an increase of the element output.
The above oxide layer is formed by a sputtering method, or by forming a metal and then subjecting the metal to oxidization in a film forming chamber or in the air. Such an oxide layer is unevenly formed, and the area in which the oxide layer does not exist serves as the sense current path. In this method, the pores in the oxide layer and the unevenness of the film thickness are used to narrow the section area of the sense current path. However, it is difficult to correct the unevenness to form a desired sense current path. This difficulty will be described below, with reference to FIG. 1.
FIG. 1 collectively shows the resistance values of samples of oxide films formed on the free layer in a spin valve film. Each row in FIG. 1 shows the element resistance of each sample in a case where sense current flows in the film thickness direction of a CPP-type magnetoresistive element. In this magnetoresistive element, a Cu (2 nm)/Ta (1 nm) metal film is stacked on the free layer of a bottom-type spin valve film, and an oxide layer is then formed by an oxygen plasma method in a sputtering film forming chamber. The oxidization process is carried out under the conditions of 350 Pa×sec. The greatest element resistance is almost 10 times greater than the smallest element resistance, despite the fact that the oxidization processes for the samples have been carried out under the same conditions. This is because the oxide layer cannot be uniformly formed for each sample. If an oxide layer is employed as a narrowing layer for the sense current path, the unevenness of a ripped oxide layer (defective portions on the inner surface of the film, pores, or oxide film thickness) should be used. These results show that correcting the unevenness of actually formed oxide layers is very difficult. Therefore, it has become apparent that further studies are necessary to secure the stability of the element properties and the reliability of products to which the above technique is applied.